Studying cell structure and Transport Recall Q

Question 1

Describe how you could make a temporary mount of a piece of plant tissue to observe the position of starch grains in the cells when using an optical microscope. (4 marks)

Answer 1

1. add drop of water to glass slide
2. obtain a thin section of plant tissue and place
   on slide
3. stain with iodine in potassium iodide
4. lower cover slip using mounted needle

Question 2

Describe and explain how cell fractionation and ultracentrifugation can be used to isolate mitochondria from a suspension of animal cells. (5 marks)

Answer 2

1. Cell homogenisation to break open cells;
2. Filter to remove large debris / whole cells;
3. Use isotonic solution to prevent damage to
   mitochondria/organelles;
4. Keep cold to reduce damage by enzymes/ use
   buffer to prevent enzyme denaturation;
5. Centrifuge at lower speed to separate nuclei /
   cell fragments / heavy organelles;
6. Re-spin (supernatant / after nuclei / pellet removed) at higher speed to get
   mitochondria at bottom.

Question 3

Describe the principles and the limitations of using a transmission electron microscope to investigate cell structure. (5 marks)

Answer 3

Principles:
1. Electrons pass through thin specimen;
2. Denser parts absorb more electrons;
3. So denser parts appear darker;
4. Electrons have short wavelengths so give high
resolution
Limitations:
5. Cannot look at living material / Must be in a
vacuum;
6. Specimen must be very thin;
7. Artefacts present;
8. Complex staining method
9. Image not in 3D / only 2D images produced

Question 4

Scientists use optical microscopes and TEMs to investigate cell structure.
Explain the advantages and the limitations of using a TEM to investigate cell structure. (5 marks)

Answer 4

Advantages:
1. Small objects can be seen;
2. TEM has high resolution as wavelength of
electrons shorter
Limitations:
3. Cannot look at living cells as cells must be in a
vacuum / must cut section / thin specimen;
4. Preparation may create artefact
5. Does not produce colour image;

Question 5

Contrast how an optical microscope and a transmission electron microscope work and contrast the limitations of
their use when studying cells. (6 marks)

Answer 5

1.TEM use electronsand optical use light;
2.TEM allows a greaterresolution;
3(So with TEM) smallerorganelles/named cell
structurecan be observed
4.TEM view only dead / dehydrated
specimensand optical (can) view live
specimens;
5.TEM does not show colourandoptical (can);
6. TEM requires thinnerspecimens;
7.TEM requires amorecomplex/time
consuming preparation;
8.TEM focuses using magnets and optical uses
(glass) lenses;

Question 6

Many different substances enter and leave a cell by crossing its cell surface membrane. Describe how substances can cross a cell surface membrane.(5 marks)

Answer 6

1.Simple / facilitated diffusion from high to low
concentration;
2.Small / non-polar / lipid-soluble molecules
pass via phospholipids / bilayer;
OR
Large / polar / water-soluble molecules go
through proteins;
3.Water moves by osmosis from high water
potential to low water potential;
4.Active transport against concentration
gradient;
5.Active transport/facilitated diffusion involves
proteins /carriers;
6.Active transport requires energy / ATP;
7.Ref. to Na + / glucose co-transport

Question 7

Oxygen and chloride ions can diffuse across cell-surface membranes. The diffusion of chloride ions involves a membrane protein. The diffusion of oxygen does not involve a membrane protein. Explain why the diffusion of chloride ions involves a membrane protein and the diffusion of oxygen does not. (5 marks)

Answer 7

1.Chloride ions water soluble/charged/polar;
2.Cannot cross lipid bilayer;
3.Chloride ions transported by facilitated
diffusion using channel/carrier
protein;
4.Oxygen not charged/non-polar;
5.Oxygen soluble in/can diffuse across lipid
bilayer;

Question 8

The epithelial cells that line the small intestine are adapted for the absorption of glucose. Explain how. (6 marks)

Answer 8

1.Microvilli provide an increased surface area;
2Many mitochondria produce ATP / release or
provide energy for active transport;
3.Carrier proteins for active transport;
4.Channel/carrier proteins for facilitated
diffusion;
5.Co-transport of sodium ions and glucose using
a symport / carrier protein;
6.Membrane-bound enzymes digest
disaccharides to produce glucose;

Question 9

The movement of substances across cell membranes is affected by membrane structure. Describe how. (5 marks)

Answer 9

1. Phospholipid (bilayer) allows movement of
   lipid-soluble substances;
2. Phospholipid (bilayer) prevents movement of
   lipid-insoluble substances
   OR
   (Membrane) proteins allow polar/charged substances to cross the membrane/bilayer;
3. Carrier proteins allow active transport;
4. Channel/carrier proteins allow facilitated diffusion/co-transport;
5. Shape/charge of channel / carrier determines which substances move;
6. Number of channels/carriers determines how much movement;
   7.Membrane surface area determines how much diffusion/movement;
7. Cholesterol affects fluidity/rigidity/permeability;